Rural housing problems in Colombia are dealt with primarily by the Credit Bank for Agriculture, Industry, and Mining, and by the Colombian Institute of Agrarian Reform. Colombia's housing shortage is largely a result of the rapid growth of the urban population. With the annual urban population growth rate at over 5%, the housing deficit was estimated to be around 800,000 units in the early 1980s and is expanding annually. Total housing units numbered approximately seven million in the mid-1990s. At last estimate, more than 85% of all housing units were detached houses, ranches, and huts; less than 15% were apartments, and the remainder were mobile units, natural shelters, and non-residential housing. Roughly 70% were owner occupied, and 25% were rented. Three-fourths of all dwellings were made of bricks, adobe, mud or stone; nearly 15% had external walls of wattle or daub; 7% were wood; and 3% were mostly cane. Public resources are channeled into urban housing through the Central Mortgage Bank, whose mortgages, because of interest rates, down-payment requirements, and repayment terms, have usually been accessible only to upper-middle-income groups; and the savings and loan corporations, whose interest rates are pegged to inflation through daily monetary correction factors known as units of constant purchasing power value (UPACs). In addition, through AID, substantial private funds from US investors have been used for various housing programs. A US $2.6-billion government program for the rehabilitation and construction of new housing in the 1987–90 period was announced by the Barco government, with 131,000 units to be constructed and 258,000 units to be improved.